1. Field of the Invention
The present invention relates to an ice island structure for location over a submerged drill site in offshore waters, and to a drilling method utilizing the ice island structure for drilling prior to grounding of the ice island structure upon the sea bed.
2. Description of the Prior Art
One method advanced in the prior art for oil drilling off the coast of Alaska utilizes a floating barge as the drilling platform. However, if the barge is allowed to become frozen into the shelf ice, drilling must be suspended until the next year.
In my U.S. Pat. No. 3,738,114, entitled "Method and Apparatus for Forming Ice Island for Drilling or the Like," issued June 12, 1973, a method and apparatus is disclosed for forming an ice island on natural ice, and increasing its mass and height until it is grounded with sufficient freeboard to enable its use as a secure and immovable base for drilling. The method and apparatus involve location of a drilling barge at the offshore drill site during a thaw period. The barge freezes in the shelf ice at the onset of winter, and concentric, spaced apart inner and outer walls are constructed around the barge on the shelf ice. The shelf ice space between the walls is repeatedly flooded and allowed to freeze until the weight of the ice mass causes it to sink and ground upon the sea bottom. Extensions are added to the walls to maintain adequate freeboard as the sinking proceeds. This securely fixes the position of the ice island so that drilling operations can take place without danger of the drill pipe being wedged or sheared away by movement of the surrounding shelf ice.
One difficulty with the foregoing method and apparatus is that construction of the ice island was necessarily delayed until the surrounding ice had frozen thick enough to support the weight of the inner and outer walls and the usual construction and assembly equipment. This shortened the period of time during the winter which could be utilized to pump in and freeze water to build up the mass of the ice island. In my U.S. patent application, Ser. No. 167,931, filed July 14, 1980, now U.S. Pat. No. 4,325,656 entitled "Apparatus and Method for Forming Offshore Ice Island Structure," I show an improved method and apparatus for forming an ice island which uses buoyant inner and outer walls which can be assembled in open water. Naturally forming ice locks in the inner and outer walls in fixed relationship and provides a base enabling layered flooding and freezing in the annular space between the walls. Layers of ice, approximately two to three inches per day, are built up to increase the mass of the ice island and gradually cause it to sink. During such sinking extensions are added to the walls to maintain adequate freeboard.
The ice mass growth or accumulation is continued until sufficient excess weight is developed to ground the island and resist the lateral forces encountered on fixed shelf ice break-up. In a typical twenty foot water depth, such excess weight is achieved with an ice island approximately thirty feet high, which would provide about ten feet of freeboard.
The desired excess weight and proper freeboard is reached much earlier in the winter season than was possible with the method and apparatus of my patent, enabling earlier use of the island for drilling or other applications.
Unfortunately, none of the foregoing systems of the prior art provide a means for drilling prior to grounding of the ice island, without exposing the drilling equipment to possible damage because of movement of adjacent shelf ice.